The reduction of leakage losses in turbines has been the subject of intensive development work for several decades. During operation of a gas turbine, relative movement between the rotor and the housing is unavoidable. The resultant wear of the housing or wear of the blades has the effect that the sealing action is no longer provided. As a solution to this problem, a combination of thick coatings which can be ground away on the heat shield and abrasive protective coatings on the blade tips is provided.
Methods for applying additional coatings to blade tips or for increasing the resistance to wear by suitable modification of the blade tip have been known even since the 1970s. Various methods have likewise been proposed for simultaneously making such protective coatings resistant to frictional contacts and oxidation caused by the hot gas by a combination of abrasive particles (carbides, nitrides, etc.) with oxidation-resistant materials. Many of the proposed methods are expensive and complex to implement, however, and this makes commercial use more difficult.
One of the popular strategies therefore lies in dispensing entirely with the protection of the blade tip against wear and providing the heat shield with special porous, ceramic rub-in coatings. Owing to their high porosity, these can also be rubbed in to a certain extent by unprotected blade tips. However, considerable technical risks are associated with this method, since the porous, ceramic rub-in coatings do not ensure the same resistance to erosion as dense coatings. A further risk lies in operational changes to the porous, ceramic rub-in coatings (densification by sintering), which can have a negative effect on the tribological properties. For this reason, a combination with wear-resistant (abrasive) blade tips is expedient when using ceramic protective coatings on heat shields.
In recent decades, a plurality of methods for producing abrasive blade tips have been developed as shown in, for example, U.S. Pat. No. 6,194,086 B1. Although the use of laser metal forming (LMF) to build up abrasive blade tips has been known since the start of the 1990s (see for example DE 10 2004 059 904 A1), this method is still used rarely on an industrial scale.